EFFECTS PEODUCED UPON BUILDINGS. 131 



the observations which have been made by various writers 

 on this subject appear to point in a contrary direction, I 

 give the following examples : — 



In the great Jamaica earthquake of 1692, the portions 

 of Port Eoyal which remained standing were situated on 

 a compact limestone foundation ; whilst those on sand 

 and gravel were destroyed (' Greological Observer,' p. 426). 

 Again, on p. 148 of the same work, we read, ' According 

 to the observations made at Lisbon, in 1737, by Mr. Sharpe, 

 the destroying effects of this earthquake were confined to 

 the tertiary strata, and were most violent on the blue 

 clay, on which the lower part of the city is constructed. 

 Not a building on the secondary limestone or on the basalt 

 was injured.' 



In the great earthquakes of Messina, those portions of 

 the town situated on alluvium, near the sea, were destroyed, 

 whilst the high parts of the town, on granite, did not suffer 

 so much. Similar observations were made in Calabria, when 

 districts consisting of gravel, sand, and clay became, by the 

 shaking, almost unrecognisable, whilst the surrounding 

 hills of slate and granite were but little altered. At San 

 Francisco, in 1868, the chief destruction was in the allu- 

 vium and made ground. 



At Talacahuano, in 1835, the only houses which 

 escaped were the buildings standing on rocky ground : 

 all those resting on sandy soil were destroyed. 



From the results of observations like these, it would 

 seem the harder rocks form better foundations than the 

 softer ones. The explanation of this, in many cases, 

 appears to lie in the fact that the soft strata were in a 

 state of unstable equilibrium, and by shaking, they were 

 caused to settle. Observations like the following, however, 

 point out another reason why soft strata may sometimes 

 afford a bad foundation. 



